1. Field of the Invention
The present invention relates to a near-field light (NF-light) transducer for generating NF-light by being irradiated with light. And the present invention relates to a head used for a thermally-assisted magnetic recording in which a magnetic recording medium is irradiated with NF-light, thereby anisotropic magnetic field of the medium is lowered, thus data can be written. Further, the present invention relates to a magnetic recording apparatus provided with the head.
2. Description of the Related Art
As the recording densities of magnetic recording apparatuses become higher, as represented by magnetic disk apparatuses, further improvement has been required in the performance of thin-film magnetic heads and magnetic recording media. Especially, in the magnetic recording media, it is necessary to decrease the size of magnetic grains that constitute a magnetic recording layer of the medium and to reduce irregularity in the boundary of record bit in order to improve the recording density. However, the decrease in size of the magnetic grains raises a problem of degradation in thermal stability of the magnetization due to the decrease in volume. As a measure against the thermal stability problem, it may be possible to increase magnetic anisotropy energy KU of the magnetic grains. However, the increase in energy KU causes the increase in anisotropic magnetic field (coercive force) of the magnetic recording medium. As a result, the head cannot write data to the magnetic recording medium when the anisotropic magnetic field (coercive force) of the medium exceeds the write field limit.
Recently, as a method for solving the problem of thermal stability, so-called a thermally-assisted magnetic recording technique is proposed. In the technique, a magnetic recording medium formed of a magnetic material with a large magnetic anisotropy energy KU is used so as to stabilize the magnetization; anisotropic magnetic field of the medium is reduced by applying heat to a portion of the medium where data is to be written; just after that, writing is performed by applying write field to the heated portion.
A technique is well known, in which the heating of a portion to be written of the medium is performed by irradiating the portion with near-field light (NF-light). For example, U.S. Pat. No. 6,768,556 and U.S. Pat. No. 6,649,894 disclose a technique in which a NF-light transducer, that is a metal plate for generating NF-light, so-called a plasmon antenna, is provided on the opposed-to-medium surface. Then, NF-light is generated by irradiating one side of the plasmon antenna with laser light guided through a waveguide, the one side being opposite to the opposed-to-medium surface.
On the other hand, the present inventors have devised a NF-light transducer in which laser light propagating through a waveguide is coupled with a plasmon antenna in a surface plasmon mode to cause excited surface plasmon to propagate to the opposed-to-medium surface, thereby providing NF-light, instead of directly applying the laser light to the plasmon antenna. The NF-light transducer has a propagation edge that reaches the opposed-to-medium surface, and the excited surface plasmon propagates on the propagation edge. The NF-light transducer is hereinafter referred to as a surface plasmon antenna. In the surface plasmon antenna, its temperature does not excessively rise because laser light is not directly applied to the surface plasmon antenna. As a result, there can be avoided a situation in which the end of a read head element, which reaches the opposed-to-medium surface, becomes relatively far apart from the magnetic recording medium due to the thermal expansion of the plasmon antenna, which makes it difficult to properly read servo signals during recording operations. In addition, there can also be avoided a situation in which the light use efficiency of the NF-light transducer is degraded because thermal fluctuation of free electrons increases in the plasmon antenna. Actually, there can be achieved approximately 20% which is the same as or more than the light use efficiency of conventional plasmon antennas. Here, the light use efficiency of a NF-light transducer is given by IOUT/IIN(×100), where IIN is the intensity of laser light incident to the waveguide, and IOUT is the intensity of NF-light emitted from a near-field-light-generating (NFL-generating) end of the surface plasmon antenna after converting the laser light into surface plasmon in the surface plasmon antenna.
The propagation edge of the surface plasmon antenna is very sharp; the condition of the propagation edge, in particular, the curvature radius of the edge has been understood to have an influence on the intensity of generated NF-light. Therefore, the condition of the edge is required to be appropriately adjusted in order to obtain a sufficient intensity of NF-light. Further, metal material which forms the surface plasmon antenna is required to have a structure with minute crystal grains so as to shape a desired sharp edge without any defects.
Silver (Ag), which is currently considered to have the highest efficiency of generating NF-light, typically has a structure of crystal grains with a variety of radii (halves of grain diameters) in the range from 20 to 50 nm (nanometers). When the Ag is used as the constituent material of the surface plasmon antenna in its formation process, there may especially occur defects in the vicinity of the propagation edge. This can cause the manufacturing process yield to be lowered. Actually, in the forming process of the surface plasmon antenna, first the whole body including the propagation edge is shaped; then, an end surface from which NF-light is generated is formed by a polishing process that determines the opposed-to-medium surface. Therefore, depending on the constituent metal material of the surface plasmon antenna, the polishing may cause some of crystal grains that constitute the propagation edge to be damaged or to drop off, thereby bringing about defects such as cracking and chipping in the vicinity of the propagation edge.
As described above, it is crucial that the surface plasmon antenna has a propagation edge in which the condition of the edge is appropriately controlled and the generation of defects such as cracking and chipping is suppressed.